When I try to compile and build a Fortran dll file, I get the message:
Unable to write to file Debug\.NET\FitSubs.dll
This file is called via a C# program in Visual Studio 2015.
In a previous version of the program, I can make changes in the Fortran.dll file and it compiles and builds fine.

To ensure I was doing things correctly, I used the example dll program in the Ftn95 help file, which interfaces with a simple Windows Form. The source code looks like this:

In the project properties the output filename is set to Debug\.NET\Project1.dll and the output file type is set to Dynamic Link Library (DLL). When I try to compile and build it I get the same error:
Unable to write to file Debug\.NET\Project1.dll